Hyperstereopsis is a undesirable effect observed when enhanced imagery of a scene is displayed to a user from sensors which are positioned at a stereo separation that is unnatural as perceived by the user. The extent of the hyperstereopsis effect is dictated by the size of the sensors used and the ability to sensibly mount such sensors to a helmet or other suitable retaining apparatus worn by the user. Current solutions for mitigating the hyperstereopsis effect include conventional optical techniques such as those disclosed in U.S. Pat. No. 5,629,807.
U.S. Pat. No. 5,629,807 illustrates a head mounted vision enhancement system that provides improved crash survivability and a better center of gravity. This is accomplished by positioning an objective lens and an intensifier tube of the system closer to a center line of the head of the user and employing an optical arrangement to bring the image back to a correct eye spacing for the user. Hyperstereopsis is thus avoided by folding the intensifier objective lenses from the center of the forehead of the user to the correct interpupil separation for the user. However, such solutions are large and heavy when installed on a helmet worn by the user.
Hyperstereopsis arises in a enhanced imagery system, for example a night vision enhancement system, when the sensors of the systems are located on the side of a helmet at a separation that is larger than the interpupil separation of the user of the system. Hyperstereopsis causes objects to appear distorted and/or closer to the user. Aviators using such systems report that the ground appears to slope upwardly towards the user and appears closer beneath an aircraft than normally expected.
Depth perception is the ability of a user to estimate absolute distances between the user and an object or the relative distances between two objects spaced from the user. The hyperstereopsis effect is a binocular perception and is the result of each eye of the user viewing slightly different images of the same object. The differences in the images occur due to the location of the sensors being different to the interpupil separation between the eyes of a user. This effect distorts the user's perception of slope in depth.
Conventional sensors, for example night vision cameras, are typically large in size and heavy. Therefore, such sensors cannot be mounted at a natural interpupil distance of a user. To do so would introduce too much mass to the front of the helmet worn by the user and could lead to clashes with a helmet visor and associated mechanisms. It will be understood that similar problems arise for head mounted displays as the mass of the sensors will be positioned such as to increase the mass on the front of the head of a user. The mass distribution of such sensors is important when used in an aviation environment wherein the user is subject to a range of gravitational forces and the possibility of a user having to eject from an aircraft whilst the sensors remain in situ relative to the head of the user.
Waveguide type displays are discussed in International publication numbers WO2007/029034 or WO2007/029032. Such waveguide displays offer the benefit of reduced mass and ease of installation in a helmet or head mounted display when compared to conventional optical displays.